1. Field of the Invention
This invention relates broadly to the field of container making, and more specifically to blow molded plastic bottles, such as the PET bottles that are in common use today for packaging soft drinks such as soda. More specifically, the invention relates to an improved footed container and bottom therefor that exhibits superior characteristics of light weighting, stability and resistance to stress cracking.
2. Description of the Related Technology
During the last twenty-five years or so, there has been a dramatic shift in the packaging of carbonated beverages, particularly, soft drinks, away from glass containers and toward plastic containers. The plastic containers initially took the form of a two-piece construction, wherein a plastic bottle having a generally hemispherical bottom was applied a separate base cup, which would permit the bottle to be stood upright. The hemispherical bottom was seen as the most desirable shape for retaining the pressure generated by the carbonation within the container. Pressures in such containers can rise to 100 p.s.i. or more when the bottled beverage is exposed to the sun, stored in a warm room, car trunk, or the like.
Such plastic containers represented a significant safety advantage over glass containers when exposed to the same internal pressures. However, the two-piece construction was not economical because it required a post molding assembly step, and, also a separation step prior to reclaiming or recycling the resins forming the bottle and base cup.
During this period of development, various attempts were made to construct a one-piece, self-supporting container that would be able to retain the carbonated beverages at the pressures involved. Such a one-piece container requires the design of a base structure which will support the bottle in an upright position and will not bulge outwardly at the bottom. A variety of designs were first attempted, with most following one of two principal lines of thought. One line of designs involved a so-called champagne base having a complete annular peripheral ring. Examples of such bottles are found in U.S. Pat. Nos. 3,722,726; 3,881,621; 4,108,324; 4,247,012; and, 4,249,666. Another variety of designs is that which included a plurality of feet protruding downward from a curved bottom. Examples of this variety are to be found in U.S. Pat. Nos. 3,598,270, 4,294,366; 4,368,825; 4,865,206; and, 4,867,323. In recent years, the latter type of design has achieved primacy in the marketplace.
Footed one piece bottles present certain problems, though, that have not yet been worked out to the satisfaction of the packaging industry and its customers. For example, the uneven orientation of the polymer in the footed area of the bottom can contribute to uneven post-filling expansion of either one or more feet or the central portion of the bottom, creating what is generally referred to as a "rocker." In addition, the presence of the feet themselves and the need to force the oriented material into the shape of the feet can create stress points in the container bottom that can adversely affect container shape. Container bottom designs that minimize stress and disorientation of the polymer during molding, then are considered preferable.
Another concern in the design of container bottoms for one piece containers is the possibility of stress cracking in the base. The amount of stress cracking is related to the geometry of the base. Relatively large radius curves in the base will reduce the potential for stress cracking compared to a base with small radius curves.
Yet another factor that is important in the design of such containers is that of positional stability after filling and pressurization of the container. It is preferable, from both a bottler's and consumer's standpoint, for a filled container to be as resistant to toppling as possible. The stability of a filled container is closely related to the radius of its "outside standing ring," i.e. the distance that the bottom contact surfaces of the feet extend from the center axis of the container.
A further factor that must be taken into account in the design of footed container bottoms is that of efficient distribution of material within the article, so that the article is as "light weighted" as possible given the necessary strength, volumetric and stability requirements of the container. Light weighting is in particular important economically for the manufacturer of the container, since it directly impacts material costs.
A need exists for an improved bottom design for a polymeric one piece container and a mold assembly for making such a container that will optimize use of material relative to strength, reduce the possibility of stress cracking, permit molding with a minimum of stress and disorientation of the polymer material, and exhibit superior resistance against toppling.